The reactivation or reexpression of telomerase (TERT) is a widespread feature of neoplasms. TERT promoter mutations were recently reported that were hypothesized to result from UV radiation. In this retrospective study, we assessed TERT promoter mutations in 196 cutaneous basal cell carcinomas (BCCs), including 102 tumors from X-irradiated patients, 94 tumors from patients never exposed to ionizing radiation treatment, and 116 melanomas. We sought to evaluate the effects of UV and X-ray irradiation on TERT mutation frequency. TERT mutations were detected in 27% of BCCs from X-irradiated patients, 51% of BCCs from nonirradiated patients, and 22% of melanoma patients. TERT mutations were significantly increased in non-X-irradiated BCC patients compared with X-irradiated BCC patients; the mutations also presented a different mutation signature. In nonirradiated patients, TERT mutations were more frequent in BCCs of sun-exposed skin, supporting a possible causative role of UV radiation. In melanoma, TERT promoter mutations were generally restricted to intermittent sun-exposed areas and were associated with nodular and superficial spreading subtypes, increased thickness, ulceration, increased mitotic rate, and BRAFV600E mutations. Our results suggest that various carcinogenic factors may cause distinct TERT promoter mutations in BCC and that TERT promoter mutations might be associated with a poorer prognosis in melanoma.